Crystallization of Mesostasis in Two Nakhlite Meteorites: the Fractal

Introduction: The nakhlites are Martian igneous cumulate rocks [1]. Two nakhlites have been investigated in this study: Nakhla and MIL 03346. Texturally, these meteorites are dominated by elongate subhedral to euhedral augite with minor olivine and intercumulus mesostasis. A simple model for nakhlite petrogenesis involves a protracted, subsurface cooling history at oxidizing conditions followed by eruption of the crystal-rich magma. The latter event is associated with crystal settling, overgrowth and partial equilibration within the lava flow [1]. It is this portion of the meteorite’s crystallization history that is addressed in this study – the cooling conditions at the Martian surface. Crystals observed in the mesostasis have shapes indicative of rapid diffusion-controlled growth under far from equilibrium conditions (swallowtail, dendritic etc.). These crystals exhibit fractal properties and may be quantified using numerical modeling techniques. Comparison of the fractal dimension, df, of mesostasis crystals between meteorites, in conjunction with previous fractal studies applied to dynamic crystallization experiments, can constrain relative cooling rates. Analytical Methods: Polished thin sections of Nakhla and MIL 03346 were initially investigated using optical microscopy (reflected and transmitted light). Detailed observation of mesostasis texture utilized a JEOL 6301 field emission scanning electron microscope (FE-SEM) at beam operating conditions of 20 kV and a working distance of 7 mm. Crystal shapes have been quantified by their fractal dimension using the correlation function technique, as outlined in [2, 3]. X-ray elemental maps and quantitative analyses were obtained using a JEOL 8900 microprobe equipped with 5 wavelength dispersive spectrometers. Petrography: Texturally, the mesostasis is distinct between Nakhla and MIL 03346 in terms of modal phase abundance, mineral assemblage and crystal shape. MIL 03346 contains the highest modal proportion (% abundance) of mesostasis (21.4 ± 5.7), while Nakhla contains significantly less (8.4 ± 2.1) [1]. For Nakhla, the mesostasis is marked by equant crystals of titanomagnetite interspersed among plagioclase laths (Fig. 1). These crystals are embedded in devitrified silicate glass. Skeletal augite and pigeonite crystals extend from large augite phenocrysts into the mesostasis. Augite also occurs as subhedral to anhedral grains between plagioclase laths (Fig. 1). In contrast, MIL 003346 mesostasis does not contain plagioclase and is characterized by dendritic olivine and titanomagnetite crystals, and anhedral silica grains embedded in devitrified silicate glass (Fig. 2). Oxides form a vermicular texture, observed at high magnification.